The ancient Greeks introduced the
study of certain kinds of curves called "conic sections." These include
ellipses, parabolas, and hyperbolas. Centuries later, Johannes
Kepler discovered that ellipses are precisely what is needed to describe
the orbits of planets revolving around the sun. Isaac Newton then
found a mathematical explanation based on his Law
of Universal Gravitation. The orbit of our Moon revolving around
the Earth is also an ellipse, as are the orbits of the moons of Jupiter
around that planet. Here are some links which will provide an introduction
to ellipses , to Kepler's Laws , and to the quantity called the eccentricity.